Not exactly Oil Spots in the truest sense, these two variations were concocted and tested along with the rest of my oilspots glazes. Both of these recipes utilize some local dolomitic limestone I found and incorporated into the recipes. If you want to re-create these, using a mix of 1/2 and 1/2 Dolomite and Limestone will get you very close.
Here’s my Lava Oil spot firing schedule, which always looked best fired in a neutral/oxidation atmosphere in a gas kiln.
You might notice that this one has a significant addition of Cobalt, half and half Custer (Potash) and F4 (Soda) Feldspars, calcined talc, and 2% Manganese. Typical that I changed too many things to give a really useful side-by-side comparison. But I suppose when I’m coming up with new variations, that’s always been my style.
Some observations on this one:
Cobalt goes a long way and pretty dramatically alters an oilspot. With a .25%-.5% addition you get a nice shift from brown and russet glaze matrix to a darker solid black glass. Beyond 1% you can get some really nice silvery qualities to the spots. The drawback is that the more you add, the more refractory the glaze tends to get – and the longer it takes for the glazes to heal.
Just spent a bit of time inputting this old recipe into Glazy.org. Pretty Sweet!
For those of you who are really interested in Oil Spots, there’s an article from 2014 that I think is worth a long look. This particular article was what got me interested in SEM microscopy when I was in Grad School:
Ancient Jian wares are famous for their lustrous black glaze that exhibits unique colored patterns. Some striking examples include the brownish colored “Hare’s Fur” (HF) strips and the silvery “Oil Spot” (OS) patterns. Herein, we investigated the glaze surface of HF and OS samples using a variety of characterization methods. Contrary to the commonly accepted theory, we identified the presence of ε-Fe2O3, a rare metastable polymorph of Fe2O3 with unique magnetic properties, in both HF and OS samples. We found that surface crystals of OS samples are up to several micrometers in size and exclusively made of ε-Fe2O3. Interestingly, these ε-Fe2O3 crystals on the OS sample surface are organized in a periodic two dimensional fashion. These results shed new lights on the actual mechanisms and kinetics of polymorphous transitions of Fe2O3. Deciphering technologies behind the fabrication of ancient Jian wares can thus potentially help researchers improve the ε-Fe2O3 synthesis.
Friend and fellow Oilspot fanatic Leonard Smith has put together some great videos on Chinese glazes. I’d highly recommend checking out his Youtube Channel and taking a look. This one shows a Chinese potter reduction cooling for iridescent oilspots!
My favorite way to systematically tweak a ceramic “Base Glaze”. John does a great job explaining it!
He can be followed on YouTube @ https://www.youtube.com/user/johnbrittpottery
And you can find more on his website @ https://johnbrittpottery.com/
Hello Again! It’s been quite some time since my last post. Gotta thank those of you who have contacted me with interest and suggestions! With so many summer projects and school stuff, it’s been very difficult to put my full efforts into any one thing… but life is what happens while you’re making plans. Anyways, enough with the excuses.
Over the summer I had the time and energy to figure out an acceptable firing schedule in our new Blaauw kilns. For as much as I love their sleek and sexy design, computer controllers, and top of the line hardware… you can’t look in the damn things while they’re firing. This poses several challenges for control freak oil spotters. Usually, the idea is to firein complete and total oxidation, going slowly through cone 7,8,and 9 to allow thermally reducing iron to bubble up through the glaze and cause the surface to crater or foam. By carefully monitoring the situation inside the kiln, and by pulling out glazed pull rings, the firer can increase the temperature slowly and fire until the glazes have significantly ‘healed over’. This isn’t really an option, so as a result a much more empirical approach was needed to find a good fit.
After 5 firings, I settled on a more or less acceptable firing schedule (the way this programming works is that the kiln starts at 0, take 1:30 to get to 200C, then 2:30 to get to 700C, etc). In Celcius;
time_temp 00:00 5
time_temp 01:30 200
time_temp 02:30 700
time_temp 03:00 1115
time_temp 02:00 1190
time_temp 02:30 1230
time_temp 02:30 1253
time_temp 02:00 1000
time_temp 02:00 500
time_temp 02:00 300
time_temp 02:00 50
time_temp 04:00 50
Once that was established, I began with some of my favorite tiles from my initial 2 rounds of oilspot base glaze recipes. My favorites:
NoCo OS: (NC)
K200 Feldspar 57.3
200m Silica 24.2
Spanish FeOx 10
Candace Black: (CB)
K200 Feldspar 60
200m Silica 20
Spanish FeOx 8
Cobalt Carb 5
Local Black Dolomite 10
Red Iron 8
Fake Mashiko: (FM)
Calcined Redart 35
Bone Ash .5
Red Iron 4
With these base glazes I began mixing, blending, and layering, and combining glazes with dipped, poured, and sprayed application. On a whim I decided to experiment with some of my manganese saturate glazes, and that’s when things started to get really interesting. There is admittedly one glaze in particular that I’m not sharing, but with a little diligence and some wet blending, a seriously motivated glaze experimenter can discover this glaze by looking at my old posting on my OSII series. Blend them all in 50/50 proportions and you’ll get the elusive but beautiful GF glaze. Hell, it might even be on my blog somewhere. That’s all I’m saying for now – I’d hate to rob anyone of the learning experience… Hah! =)
Recently I was contacted by the British potter Allen Richards who has done some pretty extensive research into lustrous gold glazes. He suggested that I try small additions of Vanadium Pentoxide. These glazes feature 2 amended manganese saturate glazes in combinations with the usual oilspot suspects.
Here are some videos of some of my latest results. None of these particular tiles have Vanadium pentoxide. As time goes by I’ll try to annotate the combinations MS corresponds to Manganese Saturate.